This article explains the meaning of area as a physical quantity. The article area (geometry) is more mathematical.
Area is a quantity expressing the size of a region of space. Surface area refers to the summation of the areas of the exposed sides of an object.
Units
Units for measuring surface area include:

square metre = SI derived unit

are = 100 square metres

hectare = 10,000 square metres

square kilometre = 1,000,000 square metres

square megametre = 10^{12} square metres
Imperial units, as currently defined from the metre:

square foot (plural square feet) = 0.09290304 square metres.

square yard = 9 square feet = 0.83612736 square metres

square perch = 30.25 square yards = 25.2928526 square metres

acre = 160 square perches or 43,560 square feet = 4046.8564224 square metres

square mile = 640 acres = 2.5899881103 square kilometres
The article Orders of magnitude links to lists of objects of comparable surface area.
Useful formulae
External links